This invention relates to a method of correcting the stator flux estimate midpoint in alternating-current systems, the method comprising the step of determining a stator flux estimate.
As generally known, a stator flux estimate representing the electric state of the machine can be used in controlling alternating-current machines. The stator flux estimate is usually determined by integrating the stator voltage vector from which the resistive losses caused by the stator current have been subtracted according to equation (1)
xcexa8s,est=∫(usxe2x88x92rs,estis)dtxe2x80x83xe2x80x83(1)
Equation (1) is expressed in a stator coordinate system in which the stator voltage vector us and stator current vector is are combined vector variables representing the voltages and currents of all the system phases, and thus the stator flux estimate xcexa8s,est is also a vector variable. In practice, the parameters of equation (1) contain errors. Integration cannot be performed completely without errors, either, and consequently the stator flux estimate will also be erroneous. Since the voltages and currents of alternating-current systems alternate sinusoidally, the vertex of the vectors calculated on the basis of the corresponding variables draws an origin-centred circle. Thus the flux determined on the basis of voltages and currents is described by an origin-centred circle.
The calculated stator flux estimate does not, however, remain origin-centred due to the above-mentioned error factors. Consequently, the stator flux estimate xcexa8s,est has to be corrected by different methods before it can be utilized for the actual control.
The variable corresponding to the flux can also be determined for other alternating-current systems, e.g. for an electric network, in which case the power to be fed into the electric network can be controlled by the same principles as an alternating-current machine. In controlling the power to be fed into the network, it is also important to obtain a realistic estimate for the flux value corresponding to the network voltage.
In controlled motor drives, eccentricity of the flux estimate or the flux is usually corrected by means of a current model drawn up for the machine, in which the stator current vector determined on the basis of the alternating currents of the machine serves as the feedback variable. In synchronous machines, the measured magnetization current and the rotor position angle can also be used as the feedback variable. The current model includes all inductance and resistance parameters of the machine and any reduction coefficients, whose accuracy determines the accuracy of the flux estimate obtained from the current model. In practice, the current model is always erroneous because of inaccurate machine parameters.
The current vector and the motor model are used for calculating the stator flux of the machine, but if the inductance parameters of the motor model are erroneous, they will cause errors in angle and magnitude in the stator flux estimate. The calculated estimate can, however, be used for keeping the stator flux vector origin-centred, although it will contain other errors.
The object of this invention is to provide a method with which the flux estimate can be kept origin-centred and which allows the correction of the flux midpoint in flux-controlled alternating-current systems in a more reliable a manner and by a simpler method even at low frequencies. This object is achieved by the method of the invention which is characterized in that the method also comprises the steps of forming the sum of the squares of the rectangular components of the stator flux estimate to obtain a reference variable, low-pass filtering the reference variable in order to obtain a low-frequency component of the reference variable, subtracting the low-frequency component from the reference variable in order to obtain a difference variable, determining rectangular correction term components of the stator flux estimate by multiplying the difference variable by the rectangular components of the stator flux estimate and a correction coefficient, and forming a stator flux estimate having a corrected midpoint by adding the correction term components to the components of the stator flux estimate.
The method of the invention is based on the idea that any eccentricity in the stator flux can be corrected using the sum of the squares of a the instantaneous values of the stator flux components as basis. Thus performed, the correction of the eccentricity can already be made at low frequencies. In addition, the correction method does not require information on instantaneous currents of the electrical machine, but only on the magnitude of the stator flux estimate.
The method of the invention thus provides the advantage of being simple and reliable as well as providing desired results already at very low frequencies.